Many traditional solvent-based cleaning applications can suffer from poor performance on aqueous born soils. A significant portion of the soils found in conventional dry cleaning can be categorized as partially or wholly water-soluble. Water-in-oil surfactants have been developed that effectively disperse water to yield optically clear homogeneous mixtures. These dispersions can effectively dissolve water-soluble soils, termed secondary solublization, if the proper water activity is achieved in a given cleaning solvent. Water activity, determined by a number of factors including temperature, the nature of solvent-solute interactions and the molar ratio of surfactant to water, is generally monitored in conventional dry cleaning by what is termed as relative humidity. A cleaning bath with low relative humidity and hence low water activity will not allow for secondary solublization of aqueous born soils. Water exceeding a critical level can lead to non-dispersed bulk water that can be deleterious to certain garment types.
Carbon dioxide based dry cleaning is a new technology that has only recently been commercially implemented. Like conventional dry cleaning solvents water-soluble soils are not inherently soluble in liquefied carbon dioxide. Surfactant systems that enable the water bearing nature of liquid carbon dioxide have been disclosed in the patent and open literature. Under certain conditions these systems have demonstrated that water-soluble materials can be dissolved and dispersed in a liquid carbon dioxide medium.
Many conventionally used water-in-oil surfactants applied to dry cleaning solvents are not compatible with liquid CO2 solvent systems. Surfactants containing what is termed to be “CO2-philic” function have been proven to be useful in the emulsification of water in CO2. The exclusive use of some of these materials can be cost prohibitive for many applications. The case for dissolution of water-soluble materials in CO2 can be further complicated by the reversible reaction between water and carbon dioxide producing carbonic acid. This weak acid which reverts back to water and carbon dioxide as pressure is lowered and CO2 is removed can have substantial implications on water activity in CO2. Lower water activity can effect the ability of the CO2 cleaning fluid to dissolve water-soluble soils. Certain pH buffers have been used in liquid and supercritical CO2 to control the pH of aqueous micro and macro-domains and in turn augment water activity. Attempts to raise the water activity in current processes by the addition of bulk water can fail because of the inability of the CO2 and surfactant combinations to sufficiently stabilize the water. Bulk water phase-separated from liquid CO2 cleaning fluids and conventional cleaning fluids can have substantial detrimental effects on many dry clean only fabrics.
Not all stains are water soluble. Indeed, a significant number of stains that must be cleaned in a dry cleaning operation are hydrophobic. Thus, in addition to aqueous detergent formulations, it is also desirable to have a means for adding low water content detergent formulations to carbon dioxide dry cleaning systems.
U.S. Pat. No. 5,858,022 to Romack et al. and U.S. Pat. No. 5,683,473 to Jureller et al. (see also U.S. Pat. No. 5,683,977 to Jureller et al.) describe carbon dioxide dry cleaning methods and compositions. Our co-pending U.S. patent application Ser. No. 09/047,013 of McClain et al., filed Mar. 24, 1998, describes carbon dioxide dry cleaning apparatus. Dry cleaning apparatus is also described in U.S. Pat. Nos. 5,467,492 to Chao et al. 5,651,276 to Purer et al., and 5,784,905 to Townsend et al.
Cleaning may present unique challenges in the fabrication of microelectronic substrates. For example, the fabrication of integrated circuits may involve tens or hundreds of processing steps. Of these steps, it has been estimated that about one in four may be a cleaning step.
As used herein, the term “microelectronic substrates” includes integrated circuit wafers, integrated circuit chips, microelectromechanical (MEM) substrates, optical substrates, optoelectronic substrates, nanotechnology substrates, other substrates that include features that are on the order of microns or less in size, and/or combinations thereof. These substrates may be fabricated from silicon, silicon carbide, gallium nitride, other single element or compound semiconductor materials, glass, metal, organic compounds and/or combinations thereof. Microelectronic substrates may include a plurality of layers thereon that may be formed by deposition, etching, sputtering, self-assembly and/or other techniques.
It is known to use liquid and/or supercritical carbon dioxide, together referred to herein as “densified” carbon dioxide, in microelectronic substrate cleaning. In particular, production of microelectronic substrates may involve multiple processing steps, many of which incorporate water as either a carrier of chemistry, or a media to facilitate the removal of process byproducts. The evolution of materials and processes has been lead by a drive toward smaller feature sizes and more complex microdevices. In some cases, the use of water in these evolving processes has resulted in challenges whereby deleterious effects of water and byproducts carried by water have been seen. The unique physical properties of densified carbon dioxide in a liquid and/or supercritical state are of particular interest in preventing certain of these pitfalls.
One such process where densified CO2 is of practical application relates to prevention of surface tension or capillary force induced image collapse. This may be of particular interest during the aqueous development of micro-lithographic images using photoresists. Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed, through a photomask or by other techniques, to a source of activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask (or other pattern generator) to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate. See. e.g., U.S. Pat. No. 6,042,997.
Capillary forces present in the aqueous drying of imaged resist patterns can result in resist deformation and pattern collapse. This problem becomes particularly serious as lithography techniques move toward smaller image nodes with larger aspect ratios. Researchers have suggested that collapse problems associated with aqueous drying will affect the 130-nm technology node, and will become more prevalent in subsequent technologies as aspect ratios increase.
Researchers at both IBM and NTT have suggested that the use of carbon dioxide in supercritical resist drying (SRD) may reduce image collapse and film damage. See, e.g., H. Namatsu, J. Vac. Sci. Technol. B 18(6), 3308–3312 (2000); D. Goldfarb et al., J. Vac. Sci. Technol B. 18(6) 3313–3317 (2000). However, while the absence of surface tension and the accessible critical temperature and pressure of CO2 have been touted as positives factors for this drying approach, the relatively low solubility of water in the supercritical phase has also been described as a challenge that may necessitate the use of chemical adjuncts to increase the transport capacity of the fluid.
Another potential problem with drying of surfaces on microelectronic substrates is the complete removal of aqueous processing, cleaning or rinsing solutions without leaving a residue, commonly referred to as a drying watermark. These watermarks result from the concentration of solutes in the aqueous processing, cleaning, or drying fluid, as said fluid is dried. In many microelectronic structures this watermark can negatively impact the manufacturing yield or ultimate performance of the device. It is desirable to have an effective method to remove (clean) water-based fluids from surfaces that eliminates the concentration and ultimate deposition of entrained solutes—eliminating watermarks.
One such challenge comes in the manufacturing of MEMs devices. Wet-processing steps generally culminate with a rinse and dry step. Evaporative drying causes water with low levels of solutes that is pooled on the surface and in various micro-features to concentrate in locations that maximize the surface area of the pool. As a result, these drying steps can lead to the concentration of once dissolved solutes in close proximity to or on motive parts. The deposited materials, which can be organic or inorganic in nature, contribute to stiction, the locking of the motive part such that it cannot be actuated. “Release stiction” as it is termed during the manufacturing step results, is believed to be derived from adhesive and Van der Waals forces and friction. The forces generated by this phenomenon can completely incapacitate motive parts on MEMs devices.
To combat stiction, manufacturers of MEMs devices use solvents such as small chain alcohols that reduce surface tension during the rinse step and facilitate a more even drying process. However, these steps alone apparently have not eliminated the occurrence of stiction. Supercritical CO2 has been proposed for drying microstructures, (see Gregory T. Mulhern “Supercritical Carbon Dioxide Drying of Micro Structures”) where surface tension forces can cause damage. Researchers at Texas Instruments Inc. among others (see, e.g., U.S. Pat. No. 6,024,801) have demonstrated that supercritical CO2 can be used to clean organic and inorganic contaminants from MEMs devices prior to a pacification step, thus limiting stiction.
Other examples of drying and cleaning challenges related to aqueous wet-processing steps come in the formation of deep vias for interlayer metallization in the production of integrated circuits. These vias, formed by methods known to those familiar with the art, typically have large aspect ratios, creating geometries that can be difficult to clean residues from. Furthermore, wet-processing steps and rinses with traditional fluids such as water leave once dissolved solutes behind upon evaporative drying. These solutes deposited at the bottom of the vias can inhibit conduction upon metallization lowering functional yields.
Systems and methods for cleaning of microelectronic structures using densified carbon dioxide also are described in application Ser. No. 09/951,247 entitled Methods for the Control of Contaminants Following Carbon Dioxide Cleaning of Microelectronic Structure, filed Sep. 13, 2001 to DeYoung et al., assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.